Soundboards/stress

[Ron Nossaman]

Hi Jim,

You're out of context here with what I tried to say. Bear with me a bit
while I attempt to clarify. My *first* point was that the soundboard isn't
just bent, like that sheet of rubber. It assumes the crown by trying to
expand with rehydration, while being constrained on one side by glued on
ribs. It's not being forced off plane by bending, it *grows*. The change of
contour isn't forced upon it, it's dimensional change is what forces the
change of contour. It is acting, rather than reacting. It isn't alone, the
ribs become part of the resulting curve. Since the dimensional change along
the length of the rib is minimal with humidity change, and it would all be
in the same direction throughout the rib (no stresses) if a panel wasn't
glued to one side of it, what bends the rib is the width increase of the
soundboard with rehydration. It's like an enormous number of little
hydraulic jacks trying to stretch one side of the rib. The rib has one side
under compression (concave), and one side under tension (convex, where the
soundboard is glued on), with a neutral stress region somewhere down it's
center. Note that the tension side of the rib is glued to the high
compression side of the soundboard. Now, how about the board?

A panel of quartersawn Sitka spruce, four feet wide at an equilibrium
moisture content of 5% will grow about a half inch at an EMC of 12%. This is
using R. Bruce Hoadley's math. If anyone else has "Understanding Wood" you
might want to check this figure out (I've blown it before). I'm assuming
these EMC figures as conservative probable limits (SWAGs). Now assume the
centerline of a 1" deep rib to be four feet long, and bowed to a 60 foot
radius curve along the centerline (for illustrational simplicity, not
theoretical perfection). Four feet represents a little over 3.8 degrees of
arc. The dimension of the convex and concave sides would be less than .02"
more, and less respectively. I don't see how that much soundboard swelling
can disappear into a space less than 1/10 it's dimension without compressing
top *and* bottom. That's in a new board.

After a board has aged and gone through a number of humidity cycles under
string load, softer wood crushes. The softer layers eventually crush enough
that they don't maintain the compression in dry cycles, go into tension
because they no longer have the natural resiliency (memory) and crack at the
weakest spot. This happens while more resilient, stronger wood is still
under enough compression to maintain crown. My point *here* is that a
soundboard is not a homogeneous mass. It's aging and failing at different
rates, in different spots, to different degrees, from the day it's
installed. When you're talking overall crown and tone production, you look
at it like a unit. When you're talking tension, compression, and cracking,
it becomes a local thing.             
If the soundboard panel was flat sawn, the crown changes with humidity
swings would be more severe, because the tangential dimensional changes are
twice the radial, but I'm not sure how it would affect later cracking. Seems
like it would be pretty tough and sound terrible.


Anyway, That's why I don't think the top of a soundboard is generally in
tension when it's new, though it will be, locally, at some time in it's life. 

I know we're talking about the same thing, just looking at it differently.

Ron Nossaman

        __________________________________________________


At 11:24 AM 6/16/97 -0400, you wrote:
>
>In a message dated 6/16/97 8:36:26 AM, nossaman@SOUTHWIND.NET (Ron Nossaman)
>wrote:
>
><<The
>reason a board under compression can have cracks is because wood isn't
>uniform in density. You are right in saying there can't be a crack where
>there is compression, but the compression isn't uniform across the board.>>
>
>
>Perhaps we need to look at the definition of compress (ion), i.e.;
>Com-press, trs v.
>1. To press together: as in compressed her lips.
>2. To make more compact by or as if by pressing.
>
>  Following the definiton of compress, and therefore the extension to
>compression, it is not possible for a structure to develop cracks in a
>surface under comperssion. Do you agree?   For structure to fail, Yes, but to
>develop cracks, No.
>
>Now let's look at tension, i.e;
>ten-sion (tin-shun) n.
>1.a. The act or process of stretching something tight. b. The condition of so
>being stretched; tautness.
>2.a. A force tending to stretch or elongate something.
>  
>  So by definition tension stretches,( pulling apart force), and cracks can
>very well happen when the failure point is reached.  Do you agree?
>
>When parallel surfaces of a given material are shaped when in a given state
>both surfaces have the same surface force applied to them (equilibrium).  Do
>you agree?
>
>  When that given piece of material is forced to assume any shape other than
> as was originally  given to it then the two parallel sides have equal but
>opposite forces applied.  i.e;  tension and compression, this applies to all
>material in a solid state only.
>
>Either a given portion of the surface of a material is under compression or
>tension it can not be under both at the same time, that would be equilibrium.
> Do you agree?
>
>  When we take a thick piece of rubber and bend it so that it forms a shape
>with a concave and a convex surface,  What happens when we slice it with a
>razor on the convex side?  What happens when we slice it with a razor on the
>concave side? 
>which side pulls away from its separated sister surface, and why ?  If it was
>Under compression would it have pulled away ?
>
>  Don't the same laws apply to a sounding board as apply to the piece of
>rubber ?
>A sounding board develops pressure ridges because it is following the path of
>least resistance in doing so.  
>  
>   In other words ridges develop because the wood cells are expanding away
>from the source of pressure (compression) and toward the point of least
>resistance (tension, pulling apart force) 
>  In following the path of least resistance the board will react in that
>manner without regard as to what is applying any resistance/force, i.e; ribs,
>belly board, rim, humidity change, etc.
>
>  As an extreme example  think of the sounding board as a ' V ' or ' < ' when
>viewed from the edge. If this soundboard  looked like this, ' || ' or ' = '
>as originally shaped,
>where would the points of tension and compression be?
>
>  Just another point of view.
>Jim Bryant (FL)
> 
>
>


 Ron Nossaman